Cap for a writing instrument



FIG. 1 is a perspective view of a cap for a writing instrument showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a bottom view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a rear view thereof; and,

FIG. 7 is a front view showing the cap in a state of use.

The broken line showing of a writing instrument is for environmental purposes only and does not form a part of the claimed design. 

The ornamental design for a cap for writing instrument, as shown and described. 